Voltage source pulse width modulated (PWM) power converters are used in many power conversion applications such as variable speed drives, wind and solar converters, power supplies, uninterrupted power system (UPS), power quality systems, among others.
Power systems employing parallel topologies are able to de-correlate the relationship between switching frequency and power capacity, thus avoiding performance constraints associated with single high power device characteristics.
Cross-currents developed within the paralleled power converters do not contribute to power transfer and can reduce the overall converter system efficiency. Additionally, since cross-currents can cause overloads of individual converters, cross-current flow can be reduced by using passive and active means.
Paralleling of the converters via direct connections of the converter outputs with no inductors used between the converters is known as hard paralleling. In hard paralleling, voltage errors, which initiate cross currents, are mainly caused by different semiconductor switch parameters and gate drivers characteristics, which can lead to switching not being performed synchronously on all converters. Due to the need for synchronous converter switching to limit cross-current flow in direct hard parallel power converters, semiconductor devices with closely matched characteristics can be used.
Alternatively, inductors can be introduced in series with converter terminals before paralleling (soft paralleling). Instantaneous cross currents, caused by non-synchronous pulse width modulation (PWM) switching, can be limited by selecting a proper inductance value. If converter switching instances are only marginally desynchronized (e.g., due to control or tolerances in the drivers and turn-on and turn-off times of the switches), the inductance used for paralleling can be very small (e.g., fraction of 1%).
Current sharing control can be achieved via local converter current control loops to ensure converter currents are equal. Alternatively, a global current controller can be used to control cumulative currents.
Each converter may use local cross current controllers to enforce sharing of currents by trimming the base voltage reference (set by the cumulative current controller). One example is to use local current loops with proportional gain (P) to electronically emulate additional internal resistance by the converter to increase effective impedance seen by the cross currents. However, using local current control loops does not take into account that, due to various current paths, inductances seen by the cumulative and cross currents are not same. The converter currents can contain differential mode and common mode (zero sequence) cross current components. In situations when the inductors are magnetically coupled, the common mode inductance (L0 seen by the common mode cross currents) and differential mode inductances (L, seen by the differential mode cross currents) may differ significantly.
Additionally, a limit exists on the P controller gain (emulated impedance), which cannot be set arbitrary high due to stability constraints. An upper limit may be approximately near one-fourth (¼) to one-half (½) of deadbeat gain L/Ts, where L is inductance and Ts is sampling period. Thus, permissible controller gains and hence error in the current control is strongly dependent on inductance the inductor used for paralleling.
Attempts have been made to improve reduction of the circulating of cross currents including incorporating integral (I) controller (i.e. with infinite gain). For example, a proportional integral (PI) controller and a synchronous reference frame (SRF) PI controller are introduced into the cross current control system (e.g., to allow for infinite gains localized around dc and positive sequence fundamental frequency). With introduction these integral controllers the cross currents at dc and positive sequence fundamental frequency can be fully suppressed to zero.
However, with this approach the negative sequence fundamental frequency and higher order harmonic cross current components can be only partially attenuated and may remain high in situations when the inductance in the cross current path is low. For example, the third harmonic in the cross current (which see only very small common mode inductance) could exceed the fundamental frequency component and would be only partly suppressed by such conventional controllers.